Wow, I just ran into this today and was quite surprised. Had to do lots of searching to find a thread that covered the topic, but I figured it had to be on here somewhere.
My 18-135mm @ 135mm compared to a 70-200 2.8 @ 135mm at ~10ft (close to MFD for 70-200), both on a 60D.
18-135mm @ 135
70-200 @ 135
IMAGE LINK: http://www.flickr.com …buckeyemark88/5629085219/
If I trust the 70-200, I guess my 18-135 is short when up close. I'd call the difference substantial. At longer distances they match. I agree with the earlier post about wanting a chart or table of FL vs. subject distance if there's that much variation.
I started out thinking this was some kind of EF vs. EF-S difference and was really scratching my head.